Printed stretchable graphene conductors for wearable technology

August 31, 2022

The combination of high conductivity, stretchability and durability make the conductors promising candidates for application in electronic devices that are worn on the skin or integrated in clothing.

PhD student Laura van Hazendonk and Assistant Professor Heiner Friedrich

Wearable technology (WT) are electronic devices that are worn on the surface of the skin or close to the skin such as in clothing. WT have life-changing applications in medicine and other fields by detecting, analyzing, and transmitting information concerning the body and/or ambient data. In addition, they are becoming an important category of the Internet of Things. Scalable production of conductive components in WT is currently achieved by printing of metal-based inks. However, highly stretchable and durable alternatives, made from skin-compatible and non-polluting materials, have not been realized. A team led by researchers from TU/e, including Assistant Professor Heiner Friedrich and PhD student Laura van Hazendonk, have developed a graphene-based ink made from environmentally friendly and skin-compatible chemicals. This research is part of the Graphene Flagship work package 9 “Flexible Electronics”. Their studies advance the understanding of graphene in liquids, the formulation of stable inks and tuning the ink’s properties to the additive manufacturing process of choice. This opens a route towards industrial production of highly stretchable graphene conductors for wearable electronics applications. Results are published now in Chemistry of Materials. 

Printing of conductors has emerged as a more sustainable, flexible, and cost-effective alternative to traditional manufacturing techniques, as material is only deposited where needed, thus minimizing waste. Printing also enables the scalable production of flexible electronics that are tolerant to mechanical bending and/or stretching. This facilitates the manufacturing of wearable electronics, which show great potential for medical monitoring applications and for the sports industry. Presently, the conductive components of printed electronics are often composed of metals. Metals are either scarce and expensive such as silver and gold or in case of copper toxic to the environment and sensitive to oxidation. An alternative to metals which would be ideal for integration into wearable conductors is graphene, which is environmentally inert, mechanically strong, abundant, and highly conductive. However, graphene-based inks suitable for the scalable manufacturing of highly stretchable, durable, and skin-friendly conductive components were not yet available.

The manufacturing of wearable electronics components sets at least three requirements to the ink formulation: conductivity, skin-compatibility and stretchability. The team used graphene nanoplatelets as a conductive filler. These platelets are produced from the abundant material graphite, a form of carbon well-known from its use in pencils. The most scalable method to process graphite into graphene nanoplatelets is through liquid-phase exfoliation, after which ink production is a natural next step. Skin-compatibility was ensured through the use of nontoxic solvents, while stretchability and substrate adhesion were achieved by adding a stretchable polymeric binder, thermoplastic polyurethane. Ink formulation consists of a few simple steps only to ensure scalability of the approach.

Wristband device and resistance vs strain curve of printed tracks demonstrating excellent stretchability and durability of the material over many cycles.

Next, the team produced conductive tracks on stretchable substrates via screen printing, blade coating and flexographic printing. The focus of the work was on screen printing, a technique widely used for manufacturing of flexible electronics. Printing resulted in stretchable tracks with a conductivity highly competitive with other carbon-based conductors. The researchers submitted the printed components to extensive stretching tests, in which the resistance response to repetitive strains was characterized. The conductors remain conductive even under 100% strain and their conductivity remains highly stable over 1000 cycles of 20-50% strain. This is the strain level found on the human skin.

The combination of high conductivity, stretchability and durability make the conductors promising candidates for application in electronic devices that are worn on the skin or integrated in clothing. Examples of such wearable technology could be wristbands for sweat sensing or pulse monitoring, on-body heaters, smart sportswear, and wearable power sources. The stretchability of the conductors developed by the TU/e-led team not only enhances comfort of these wearables, but also improve their lifetime. The scalability of the ink preparation and printing processes bring high-volume production of stretchable wearable technology into scope.

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